This invention relates to a device for determining position coordinates of points on a surface, and more particularly to such a device commonly called a cordless digitizer, which has a cursor (or cursors) not connected to a processor by an electric cable.
The prior arts of this invention are disclosed in U.S. Pat. No. 4,848,496, "COORDINATES INPUT APPARATUS" and U.S. Pat. No. 4,786,765 "COORDINATES INPUT SYSTEM". In these prior arts, a same grid winding in a tablet is used as a driver winding and a sense winding by time division system. A cursor has a tuning circuit which is tuned to the frequency of the current in the grid winding and maintains the current in the tuning circuit during the time when the driving current in the grid winding is suspended. This current in the tuning circuit induces a voltage in the grid winding. This voltage in the grid winding during the quiescent state is detected to determine the position of the cursor relative to the grid winding.
The grid windings are constituted of a plural number of equally spaced X-windings and a plural number of equally spaced Y-windings. Each X-winding is a one turn coil comprising a pair of parallel conductors laid in X-direction, and each Y-winding is a one turn coil comprising a pair of parallel conductors laid in Y-direction, Y-direction being perpendicular to X-direction.
The X-position of the cursor can be determined by the amplitude variation of the detected voltages in successive Y-windings, and the Y-position of the cursor can be determined by the amplitude variation of the detected voltages in successive X-windings.
Since the current in the tuning circuit of the cursor must be maintained during the period when there is no external excitation, the Q of the tuning circuit must be high (in other words, the damping must be low) and the amplitude detection must be completed in a short time. These special requirements in these prior arts come from the drive-sense-duplexing in which a same grid winding is used for a driver winding and a sense winding.